Papua New Guinea corruption investigator Sam Koim says Australia should take a greater interest in the case against his country's Prime Minister Peter O'Neill.

Mr O'Neill was issued with an arrest warrant last week after an investigation by the now defunct anti-corruption body, Taskforce Sweep, into alleged multi-million dollar payments to a law firm.

Mr O'Neill has denied the charges, claiming they were politically motivated, and responded by disbanding the anti-corruption body as well as sacking the attorney general and deputy police commissioner.

"We basically made an assessment of the case and we found that Mr O'Neill knew that the bills were improper and he dishonestly directed the bills to be paid to a law firm that did not deserve those bills," Mr Koim

"The evidence was tested and corroborated independently and we reached a firm belief that there is a case."

Mr O'Neill says the letter authorising the payments is a forgery which did not originate from his office.

Mr Koim said the investigation suggests between $20 million and $25 million was paid by PNG's Department of Finance as a result of the directive, with some of the funds making its way to Australian bank accounts.

He says while he's seeking Australia's backing for his investigation, he's not suggesting it should withdraw support for Mr O'Neill.

"The call (on) whether Australia should take an action or not is it's Australia's call," he said.

"I cannot ask the Australian Government or demand Australia to do that.

"But for us, we have a strong case to pursue. It's not only my own ... different authorities are involved in separately reaching one and the same conclusion."